America’s epidemic of untied shoes can be blamed in part on faulty shoe-tying technique, according to rigorous experiments in a newly published study. But there is also a more insidious culprit: The mere act of walking loosens even a beautifully knotted lace and quickly leads to the grim fate the study’s authors call “catastrophic knot failure.”

“What was remarkable to us was how fast it happened,” says study co-author Oliver O’Reilly of the University of California at Berkeley. In treadmill experiments, after several minutes with no apparent footwear malfunction, “it took two strides for the shoelace to untie. … It explains why when you’re walking along and everything seems completely fine, all of a sudden, boom!” – you’re tripping on your lace.

Through a series of experiments, the team worked out the undoing of a shoelace. The downward spiral begins with the foot’s repeated impact against the ground, which loosens the central knot. Meanwhile, the leg’s swinging causes the lace’s free ends to whip around and gradually slide out of the knot. Finally, one lace end slips free, resulting in “runaway knot failure,” the researchers write in this week’s Proceedings of the Royal Society A: Mathematical and Physical Sciences.

The researchers developed this scenario partly by watching the shoes of a runner on a treadmill. Then they rigged up a pendulum whose swinging arm hit a platform, re-creating the foot striking the ground. They tied shoelaces to the pendulum arm and weighted the laces to mimic the forces of the swinging leg. The results confirmed it takes both the foot impact and the lashing laces to unravel the knot.

The testing showed not all knots are equal. Shoelaces tied the conventional way, using the "bunny ears" technique taught to kids, failed every time they were tested at the maximum weight. The team labeled this the “weak knot.” But the so-called “strong knot” came apart in only half of the 15-minute lab trials at the maximum weight. (To make a strong knot, cross the left lace over the right and pull it through the resulting loop. Form both the right and the left lace ends into loops and wrap the bottom of the right loop around the bottom of the left.) The team did not look at double-knotting.

The weak knot’s pathetic performance is no surprise to knot theorist Colin Adams of Williams College, who was not part of the study. The weak knot, he notes, is a version of the “granny knot,” made by crossing the left end of a piece of string over the right, pulling the left end through the resulting loop, and repeating. The researchers’ strong knot is a version of the “square knot,” made by crossing left over right as before but crossing right over left the second time.

The evidence for the granny knot’s inferiority has been mostly “anecdotal,” Adams says. The new study “is proving the fact that the granny knot is a loser and the square knot is the way to go.”

The results are not just good entertainment. “To this day we do not have a rigorous scientific understanding of the more complex types of knots,” says Khalid Jawed of Carnegie Mellon University, also not involved with the study. The new results, he says, provide “a strong first step” toward such an understanding.

The research may enrich science, but it may not revolutionize shoe tying. “It’s a little embarrassing,” says Berkeley’s Christopher Daily-Diamond, a study co-author who ran the hundreds of pendulum tests and says it’s been hard to undo old habits. “Even with all the work I’ve done with this … I still, most of the time, tie the weak knot.”